1) Field of the Invention
The present invention relates to maize plants having increased seed yield, and more particularly, to a method for producing inbred and hybrid corn plants having the phenotypic property of expanded corn ear tips for providing increased seed yield based on a recessive genetic system for trait selection.
2) Description of Related Art
Corn is the most extensively grown of all grain crops in the United States. It is of great agricultural and economic interest to provide new corn inbreds and hybrids which display an improvement in particular characteristics, such as increased seed yield, disease resistance, standability, tolerance to environmental factors, and the like. Through proper breeding techniques, these characteristics can be introduced into new or existing inbred lines of maize which can then be used to produce superior hybrid corn, which is the predominant commercial type.
Many years ago plant biologists and naturalists noted that when diverse strains of maize were crossed or hybridized, their vigor increased. This response became known as hybrid vigor or heterosis. In focusing on increased seed yield, prior efforts in heterosis and maize genetics led to a treatment of increased seed yield as being due specifically to numerous dominant and additive gene actions for beneficial effects on kernel weight, kernel number per ear, and ears per plant and unit land harvested.
In modern breeding, the complexity of corn genes affecting yield have been largely discussed under the heading of QTL's or “quantitative trait loci.” The entire corn growing ecosystem and plant response has since been modeled into various genetic determinants (see J. T. Richie and G. Alagarswamy, Agronomy J. 95:4-9, 2003. Am Soc. Of Agronomy). However, the specific manipulation of determinants or a single determinant in a complexity of gene interactions is very difficult. It would be extremely useful if one could change the hybrid corn plant ear to increase kernel number per ear without altering the remainder of beneficial traits of a hybrid such as heat tolerance, disease resistance, fertilizer efficiency and so forth.
In corn, parent strain selection for higher yield in hybrid crosses has mainly involved large numbers of inbred lines being formed and their subsequent testing as hybrids for heterosis toward higher yield. Plants that have been self-pollinated and selected for type for many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny. A cross between two homozygous lines produces a uniform population of hybrid plants that may be heterozygous for many gene loci. A cross of two plants each heterozygous at a number of loci will produce a population of hybrid plants that differ genetically and will not be uniform.
The development of superior corn hybrids requires the development of homozygous inbred lines, the crossing of these lines, and the evaluation of the crosses. The goal of corn breeding is to develop new, unique and superior corn inbred lines and hybrids. In pedigree selection breeding, the breeder combines the genetic backgrounds of two or more inbred lines or various broad-based sources into breeding pools from which the new inbred lines are developed by selfing and selection of the desired phenotypes. The new inbreds are crossed with other inbred lines and the hybrids from these crosses are thoroughly tested and compared to appropriate standards in environments representative of the commercial target area(s).
Pedigree selection breeding starts with the crossing of two genotypes, each of which may have one or more desirable traits or more desirable characteristics that are lacking in the other or which complement the other. If the two original parents do not provide all of the desired characteristics, other sources can be included in the breeding population. In the pedigree selection method, superior plants are selfed and selected in successive generations. In the succeeding generations, the heterozygous condition gives way to the homozygous lines as a result of self-pollination and selection.
Hybrid vigor has been the major means to enhance yield for more than 75 years. The general hypothesis behind heterosis is that the genetic loci on the DNA strands of the ten chromosomes of maize cooperate better when they are from different parents of different composition. Inbreeding has been found to specifically reduce seed yield.
In corn, the product of inbreeding is termed an “inbred” or “inbred line” of which there have been hundreds of thousands of bushels sold or employed as parents for hybrids. Inbreeding is typically accomplished by self-pollination as described above. At each self-pollination the number of heterozygous progeny loci is reduced by 50%. Eventually, after several generations of selfing, a progeny row from a single ear will appear very uniform and individuals are almost indistinguishable to the eye. Generally, after 6-10 selfings, a line is typically considered ready for use in hybrid manufacturing because the character of the plant is highly reproducible at that point.
Backcrossing is another means of inbreeding and inbred production. Four to six backcrosses of a random corn plant by an inbred may produce a uniform inbred population with the traits of the recurrent parent, except for those traits selected as different from the recurrent parent in each generation of backcrossing. For recessively inherited traits, after each backcross a cycle of self-pollination is useful. Inbreeding a heterozygous plant or population or backcrossing an inbred to a heterozygous plant or population is known to result in decreased yield. This has been attributed to the uncovering of deleterious recessive genes or breaking up a cooperative beneficial heterozygous loci.
In commercial breeding when two candidate inbred parents are crossed and fail to show hybrid vigor, that combination is discarded as a hybrid candidate. Inbred parent seeds are marketed based on units of 1000 viable kernels or MVK. Thus, for an inbred, a means of increasing seed number per harvested whole ear is an important matter effecting cost of parent seed production. “Seeds per acre” is also an important component of hybrid yield.
There are several thousand genes in the maize genome and these are forming as the genome evolves each year through both natural and artificial means. Some studies relating to the genomics of yield have concluded that most traits of interest to the maize breeder are strongly affected by the environment and necessitate complex and costly experimental designs for their definition. The concern of much of the genome research is with the identification and manipulation of traits which are effected by several genes, the so called QTL's. Interim results have indicated that large numbers of small loci may effect such a parameter as the yield×environment interaction. (see S. J. Oppenshaw and E. Frascaroli, Proceedings of the 52nd Annual Corn and Sorghum Research Conference 44-53. 1997. QTL detection and marker assisted selection for traits in maize).
It would thus be desirable to isolate genes in inbred lines which affect a valued component of yield without such multi-locus complexity. At the present time, one cannot simply pick out a gene and then increase it per se in any average maize plant to give a higher seed yield result. Among the problems in doing this is that the existence of a unit of DNA does not ensure its expression in the complex milieu of a plant. Single genes which affect yield by increased disease resistance are known but these are generally regarded as auxiliary traits not directly reading on ear or plant morphology. Multi-gene traits such as vertical leaf habit or strong stalks can be considered a component of yield. A simply inherited genetic determinant has not been identified which in isolated use determines a quantitative result for increased yield in hybrid form until the present invention.
What makes finding determinants to increase yield difficult for a person skilled in the art is: 1) development and isolation of pure breeding strains carrying the trait, requires several years; 2) crossing the inbreds to achieve pollen and silk nick without interference from outside elements; 3) harvesting the viable hybrid seed; 4) planting and succeeding in growing out the progeny in such a manner that phenotypic traits of the hybrid can be measured. The current products of the hybrid maize breeder, as to the shape and form of the hybrid maize seed bearing ear (the female inflorescence or flower), fall into one of the following simple descriptions or categories: 1) largely parallel side ears as a result of the seeds occurring on parallel sided cobs with the butt of the ear nearest the stalk shaped in a blunt manner and slightly larger in diameter than the distal tip of the ear; 2) slightly conical or moderately conical ears with the distal tip of the ear decidedly smaller than the base of the ear, and the tip of the ear having fewer kernels than the base, with the sides of the ear not parallel. Ears of tropical and exotic maize have been reported with various shapes; however, none of the heritable determinants or genes have been employed to make a controlled ear morphologhy with useful yield effect in a subsequent single cross hybrid, backcross hybrid, or double cross hybrid.
The ramosa allele is an example which modifies maize ears by various split shapes in tropical varieties where it was described. Small split ears or furcated ears were derived from the variety Quicheno Ramoso and their origin was the McBryde Collection #26 and #47. A practicum for the use of the ramose gene in maize in finding a single recessive genetic mechanism for increasing maize yield improvement has yet to be presented. Further, there has not been any suggestion that hybrid or inbred maize ears with useful modified ears could be developed from crosses with standard maize plants. Accordingly, there has not been any described or implemented practical means of using the ramose type effect for enhancing plant yield.
The prior art fails to disclose any prescribed use of the heritable genetic determinants known to effect the morphology of the corn ear tip in the improvement or management of plant seed yield. Thus, the commercial production of maize as a single cross hybrid, three-way cross hybrid, backcross hybrid, double cross hybrid or parent inbred has not been accomplished wherein the tips of the corn ear, or a large percentage of ears, in a field or row of plants employed commercially, are routinely larger than the base or the mid portion of the ear, and wherein the tip of the ear contributes significantly more to yield than a normal shaped ear in the same family.
Heretofore, maize breeders have neglected the intentional manipulation of ear tip phenotypic traits through recessive genetic determinants to provide for expanded ear tips. All currently employed hybrids and open pollinated varieties lack enlarged ear tips in any significant percentage of their population. Such a feat as to be able to routinely make hybrids with a large percentage of the plants having expanded ear tips will provide more harvestable tissue mass per acre planted.
Accordingly, it is the object of the present invention to provide a method for producing a hybrid and inbred corn ears having an expanded ear tip larger that the base of the ear which results from the selection of recessive trait(s) in the breeding process, said traits when combined in the hybrid result in enhanced seed yield.